I am wondering if someone can help explain portable battery chargers and how to select an appropriate sized one. I realize how many charges you want to get out of it will affect what size you want, but I'm not sure I understand how to relate mAh ratings to how many charges you can expect to get out of it.

For example, I have the New Trent IMP70D 7000mAh which says on their website "Capacity 7000mAh@5V, 3.5 times of iphone 4 battery life." I believe an iphone 4 has a 3.7V 1420 mAh Li-Polymer battery so my untrained brain thinks 7000 ÷ 1420 = 4.9 not 3.5. But okay, you're not going to bring the portable charger down to 0 so maybe that's why it's 3.5? The remaining mAh is just a reserve you can't tap?

So then I'm wondering how many charge times should I expect from it with my 1780 mAh battery - maybe 2.5 (7000 ÷ 1780 = 3.9, minus the reserve)? But since I don't run my phone down to 0 either, then I should really get maybe 5 "top offs" if I only let it get to 50%? Or doesn't it work like that?

As you can see I'm struggling with some of the basic concepts here. I know there's been some threads about which portable charger to buy and I thought that others might also find a little primer helpful as they evaluate different mAh options. I've just gotten the New Trent and only had the chance to fully discharge it (or at least discharge it so the lights don't come on anymore) only once, but I did not get 5 top offs of my Razr. I'm wondering if that will improve over time or if my expectation is wrong.

your charging a battery with a battery so there will be a good bit of lost potential on long charging cycles.
the longer you charge the more heat is generated, heat degrades battery life, therefore keep the charge cycles short as possible for longest life.
optimally, you would get the best performance (longest amount of total hours) if you kept the external plugged in all the time. the phone would cycle short charges as needed and cause far less degradation in the external caused by long high load discharges.

im sure there is a formula only a rocket scientist would understand somewhere, its no where close to a direct division though.

also, when your phone battery reports 0 its not really dead, it is simply reported as 0 by the phone because it is at the point where the voltage is too low to sustain the hardware (usually less than one volt below the maximum charged voltage).
the same is not true for the external, that will discharge to the point at which the voltage is equal to that of the battery in the device you are trying to charge.
at that point, theoretically, you could leave the external plugged into the device and they would discharge equally until the minimum operating voltage was reached in the device, thus using power from the external that would normally be left unsued by unplugging it when its not able to gain charge on the device.

I ran two tests, each time I discharged my phone to ~50% and then connected it to the fully charged 7000mAh power pack. Both times it was able to top off the phone to 100% the first time but it was not able to top it off again.

So without recharging the pack it could only top the phone off from 50% once and after that it got to 75-80% and was done. When I put the power pack on the charger none of its LEDs lit up and it began charging like it was depleted.

Not sure if it was defective but that seems unacceptable for a 7000mAh pack to me.

So I'm still looking if anyone has a recommendation.

And I'm still trying to understand this all better too, if someone (hey Foxkat, you there? ) can help.

Premium Member

I ran two tests, each time I discharged my phone to ~50% and then connected it to the fully charged 7000mAh power pack. Both times it was able to top off the phone to 100% the first time but it was not able to top it off again.

So without recharging the pack it could only top the phone off from 50% once and after that it got to 75-80% and was done. When I put the power pack on the charger none of its LEDs lit up and it began charging like it was depleted.

Not sure if it was defective but that seems unacceptable for a 7000mAh pack to me.

So I'm still looking if anyone has a recommendation.

And I'm still trying to understand this all better too, if someone (hey Foxkat, you there? ) can help.

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Hehe... yeah, I'm here. First, the former poster mentioned heat as being detrimental to batteries which is true. Still, I don't believe the battery's build in temperature sensor will allow the battery to reach temperatures that are detrimental to itself. The protection circuitry will interrupt the charging and allow the battery to cool down before allowing charging to resume.

As for the reason the charger is 7,000ma and yet still only rates 3 to 4 charges likely has more to do with rated capacity versus available useable charging capacity. The previous post was a good explanation. Charging from one battery to another requires that the charging battery has a higher voltage. It's not much different than pressure. If you want to refill a vessel to a specific pressure you need to fill from another vessel that has a higher pressure than you want to end up with. In order to end up 1,780ma at the fully charged voltage level in the RAZR you need to have a source that can supply a constant voltage that is higher than the fully charged voltage in the RAZR and also have at least the same 1,780ma left over after charging is complete. In other words, the first 1,780ma in the charger is a reserve.

Now, there are ways to accomplish the above and do it with a battery that has the same (or even lower) voltage through circuitry that steps up the voltage using available current (amperage). Battery powered LED flashlights now use this technique to maintain the voltage supplied to the LED so the light output remains constant even while the voltage from the batteries is falling during discharge. Stepping up voltage consumes more current so the rate current is used increases as voltage drops.

In this way, you could take advantage of the reserve current and get perhaps more charges out of the same charging battery. Whether the charger you have is a step up charger or not is the question.

Also, some loss is incurred to heat both through the stepping up of the voltage and due to the warming of the battery being charged. In the end, you won't get the full capacity of the charging battery in charges on the battery to be charged. As to why your test results were disappointing, it may be that the charging battery doesn't have a step up circuitry or perhaps it does but still doesn't raise the voltage high enough to accomplish a full charge on the RAZR. Otherwise it may be defective, though odds are that it isn't.

The OEM charger puts out 5.1 volts at 750ma yet it charges the RAZR battery to 1,780ma at 4.2 volts. As long as the voltage is higher from the charger than is in the battery, it will continue to supply current and charge the battery until the voltages equalize or until the battery fails. This is why there needs to be circuitry that monitors the voltage in the RAZR while charging to prevent overcharging.

I'll have to do some research on the charger you have to get a better understanding of why you got the results you did. I will likely get one for myself eventually so I'll want to know how well it works.

That helps me understand the concepts better although it does seem like the charger should have been capable of at least two 50% top offs. I suspect it was defective as they have gotten good reviews. So I think it's a good company.

I sent it back and waiting for someone to post with better results and say what they're using. I don't need something for a week long camping trip but something that can give a couple of charges would be great.

Premium Member

That helps me understand the concepts better although it does seem like the charger should have been capable of at least two 50% top offs. I suspect it was defective as they have gotten good reviews. So I think it's a good company.

I sent it back and waiting for someone to post with better results and say what they're using. I don't need something for a week long camping trip but something that can give a couple of charges would be great.

Thanks again for your insight. I was hoping you'd jump in.

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Your expectations are quite reasonable so the fact that it didn't work warrants at least a call to the technical support department.

Hehe... yeah, I'm here. First, the former poster mentioned heat as being detrimental to batteries which is true. Still, I don't believe the battery's build in temperature sensor will allow the battery to reach temperatures that are detrimental to itself. The protection circuitry will interrupt the charging and allow the battery to cool down before allowing charging to resume.

Sent from my DROID RAZR using DroidForums

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i said "your charging a battery with a battery so there will be a good bit of lost potential on long charging cycles.
the longer you charge the more heat is generated, heat degrades battery life"

the battery doesnt have to reach detrimental temperatures to lower the stamina of a battery.
if you take a battery thats 70deg and one thats 100deg, you will get more life out of the 70deg battery every time.
elevated temps lower the over all value of a battery, its a known fact.

Premium Member

i said "your charging a battery with a battery so there will be a good bit of lost potential on long charging cycles.
the longer you charge the more heat is generated, heat degrades battery life"

the battery doesnt have to reach detrimental temperatures to lower the stamina of a battery.
if you take a battery thats 70deg and one thats 100deg, you will get more life out of the 70deg battery every time.
elevated temps lower the over all value of a battery, its a known fact.

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You are correct, and I didn't mean for my post to sound like I was discounting your comment. Perhaps you missed my comment further on... "The previous post was a good explanation.", referring you your post.

Certainly heat is detrimental to virtually all electronics, and Lithium based batteries do in fact lose certain increasing percentages of their ability to hold charge over time, even under normal use. Also, heat will essentially accelerate that process. Still, for practical purposes the potential losses would be miniscule from the durations of exposure to heat from the infrequent cycling of charges.

Also, the charging circuitry monitors the temperature of the battery and knows what levels are detrimental, so it shuts down the charging process (or more accurately limits it) while monitoring the temperature until the battery temperature reverts to a "safe temperature range" at which point the charging circuitry resumes the charging at whatever rate it was at based on where in the charging process it was at that time. However to your point, at least one study (performed by Battery University) has shown that constant exposure to excessive heat (i.e. 24/7/365) will produce capacity losses ranging from 10% to 15% of initial rated capacity for temperatures elevated to 40C (104F) for one year versus the same period at 25C (77F). In other words, it's not so much that the temperature goes up, but more so for how long it remains up that is the catalyst for battery degradation.

I believe that the infrequent and short durations of potentially elevated battery temperatures SallyC will experience using the battery to battery charging process as opposed to the other options is not likely to have any appreciable impact on the useable lifespan of her battery, and the fact that she is charging more frequently (a known good) rather than less may actually offset any temperature-based loss impact as well. Interestingly enough, keeping the batteries at full charge rather than partial charge for extended periods of time also has a similar degrading effect as the chart shows (again a testamentary to short charging cycles for anyone listening), and a combination of heat and a full charge has an even greater detrimental effect.

I do have one comment, that is...it's not the length of the charging as you mention, so much as the RATE of charge that causes heat. In other words, it is entirely possible to charge a battery over even days rather than hours and produce very little increase in temperature. You simply need to reduce the voltage level of the charging rate or limit the current. On the other hand, even a short charging cycle (i.e. 10 minutes) from a "rapid charger" (which typically push higher voltages and have greater current limits), can elevate the battery's temperatures quickly and potentially into the ranges we've been discussing here. Even a car accessory rapid charger will cause the battery on the phone to heat up quicker than the wall-wart charger (charging block) that ships with the phone.

You did also mention another factor for prolonging battery life than I have made many posts regarding...the length of charging cycles and the recommendation for refraining from cycling between full discharges and full recharges. To quote you; "therefore keep the charge cycles short as possible for longest life." The fact is that Lithium based batteries will yield much longer service life if charged more frequently and for shorter "top off" or "interim boost" charges rather than 0% to 100%. I am a huge advocate of plugging in whenever there's ability to do so - whether it be a portable charging pack as SallyC referred to, a wall-block that you carry with you, a car accessory adapter, a docking station or car navigation dock, or even just the USB port on a desktop PC, for instance. No matter what your source of choice, your battery will last longer if it is more frequently replenished in shorter bursts rather than long overnight charges with no suplemental charging through the day.

To recap, I wasn't disagreeing with you, more the contrary - but I was trying to look at the situation from a more practical, real-world point of view. The article mentioned above also discusses using those wireless charging mats and suggests that they may also add to the degradation over time, for the same reason you mentioned...increased heat. Again, I have to question though, how much real capacity loss would be incurred over time, and would it be notable or essentially undetectable under normal use. I think that in a real-world scenario, the lifespan of the battery would be impacted minimally and it's more likely that the phone would be replaced before the effect from the increased temperature would be notable. Don't forget, there's also the normal decreasing capacity that happens over time and also by the number, starting and ending charge levels and durations of charging cycles, so without a true study, no one will know for sure.

I got the PowerGen UltraSlim from Amazon for $26. Although it is only 2900mAh it is only 10.5mm thin and about 3/4 the length and width of my RAZR so it is much easier to carry in my pocket. It has a built-in micro SD cable to charge my phone and works well for the occasional need to provide some spare juice when out and about.

I got the PowerGen UltraSlim from Amazon for $26. Although it is only 2900mAh it is only 10.5mm thin and about 3/4 the length and width of my RAZR so it is much easier to carry in my pocket. It has a built-in micro SD cable to charge my phone and works well for the occasional need to provide some spare juice when out and about.

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I've heard good things about it. The size is a big plus. Can you get a full charge?

As far as I know it is fully charged. There are 4 blue led's that light up in sequence (25% - 100%) as it is charging to show the % charged level and it eventually stops blinking with all 4 led's solidly lit.